1Heat oven to 425º. Grease 2-quart gratin dish or shallow baking dish.
2Heat milk, garlic, salt and pepper to simmering in 12-inch skillet over medium-low heat; add potatoes and leeks. Cook uncovered about 15 minutes, stirring frequently, just until tender.
3Spoon potato mixture into gratin dish. Mix sour cream and mustard; spoon over potato mixture. Sprinkle with cheese. Bake uncovered 20 to 25 minutes or until bubbly around edge and golden brown on top.